There is a demand in principle for electrically conductive structures on surfaces of objects with poor surface conductivity. With regard to the conductivity, for example, uses in the integration of electronic circuits into an electronic component by impressing conductive material on the surface of the component are desirable. Costly composite problems of components with separate circuits could thereby be minimized. In particular, the printing of surfaces of flexible materials with electrical strip conductors is particularly interesting. The freedom of design of the whole component with a flexible content should no longer be influenced by the circuit provided.
The application of copper strip conductors is known. These, however, are applicable to surfaces only with costly deposition and etching processes. Electrically conductive pastes (e.g., conductive silver) which can be subsequently applied to surfaces and are used for contacting, are a further development.
There is particular interest in printing polymer materials. During the printing process by which the surface is made conductive, the surface of the substrate should not be heated above the softening point (e.g., glass transition temperature of a polymer surface) of the surface material. In addition, no solvent that dissolves or partially dissolves the surface may be used.
Known processes with which structures can be applied to surfaces inexpensively and with good throughput, are screen printing or offset printing processes. These two processes, however, place further requirements on the printing substance used. Thus it is known to the person skilled in the art that inks or dyes that should be used with these printing processes place minimum requirements on the viscosity of the printing ink. The viscosity must be in the range above 1 Pa·s so that good printing results can be achieved.
U.S. Pat. Nos. 5,882,722 and 6,036,889, the entire contents of each of which are hereby incorporated herein by reference, describe conductive formulations which contain metal particles, a precursor and an organic solvent and which only form conductive structures at a sintering temperature from 200° C. upwards. These known formulations have a viscosity of approximately 10 Pa·s. The formulation can in fact be used for the printing technologies described (screen printing, offset printing), but because of the high sintering temperature required, use for application to surfaces of polymers is restricted.
International Patent Pub. No. WO2003/038002 and U.S. Pat. App. Pub. No. 2005/0078158, the entire contents of each of which are hereby incorporated herein by reference, disclose formulations with silver nanoparticles which are stabilized inter alia with sodium cellulose methyl carboxylic acid. These documents in fact describe the need for post-treatment, e.g. by heat, or flocculants, but not the processing temperature or the conductivity of the microstructures obtained from the formulation. Furthermore, the accurate distribution of the nanoparticles used and obtained is not disclosed, although the size range is less than 100 nm. The content of silver particles of the disclosed formulations is not more than 1.2 wt. %. The viscosity of the printing formulation typically necessary for the inkjet process provided is approximately 10 mPa·s. The formulation is therefore not really feasible for screen or offset printing.
European Pat. Pub. No. EP 1586604, the entire contents of which are hereby incorporated herein by reference, discloses a silver paste that is composed of an epoxy resin, silver flakes and silver nanoparticles. This paste forms a conductive film after printing on or application to the surface of a base material and subsequent heat treatment. Resistances of less than 5×105 ohm/cm are achieved at sintering temperatures above 200° C. This high sintering temperature greatly restricts the selection of the printable polymer substrates.
International Patent Pub. No. WO 2008/031015, the entire contents of which are hereby incorporated herein by reference, discloses an aqueous formulation which also contains silver flakes. Conductivities of 0.022 ohm/square can be achieved with this formulation at 120° C.
HARIMA offers the product line “NP Series Nano-Paste” which is a nanoparticles-based silver conductive ink with low viscosity. HARIMA, however, gives sintering temperatures of 210-230° C.